1. Field of the Invention
The invention is directed to method for producing a line of weakening by means of a laser such as is known from DE 196 36 429 C1 and to an airbag cover produced by this method.
2. Description of the Related Art
The invention is directed to method for producing a line of weakening by means of a laser such as is known from DE 196 36 429 C1 and to an airbag cover produced by this method.
The prior art discloses many airbag covers (hereinafter: cover) designed and produced in different ways. All of them have the object of safely preserving and protecting the airbag and, in case of activation of the airbag, releasing within milliseconds an opening through which the airbag can deploy and protect the passengers of the vehicle.
The trend in recent years has tended toward covers which are not visible to the passengers. Such covers are produced in that a line of weakening which defines the subsequent opening is generated in the cover (which generally comprises a rigid substrate layer, a foamed material layer and a trim layer or decorative layer) or in individual layers of the cover.
The difficulties involved in generating such lines of weakening with different techniques are discussed in EP 0 711 627 A2. It is mentioned that the scoring depth of the line of weakening must be carefully controlled in order to cause a reliable rupture of the outer cover layer (decorative layer) at exactly the right moment. On the one hand, the depth of the score should not be too shallow so that the resistance to be overcome in opening is not too great; on the other hand, sufficient material should remain so that the line of weakening is not outwardly visible. It is also mentioned that in cover layers having an irregular inner surface the scoring depth must be controlled in such a way that the remaining residual wall thickness is constant. By way of solving this problem, it is suggested in EP 0 711 627 A2 that a continuous groove be cut in by means of a laser beam, this laser beam being controlled to achieve a constant thickness of the material remaining below the groove and to achieve a constant groove depth.
In order to generate lines of weakening with different resistance to opening, the groove can be generated with different depths. At all of the depths mentioned by way of example, the substrate layer is completely penetrated and the foamed material layer is penetrated at least partially or is penetrated completely into the decorative layer. Since the groove is continuous in every case, excepting the above-mentioned alternative line of through-holes, which anyway does not meet the object of invisibility of the line of weakening, the residual wall thickness remaining below the groove can not be minimized to an unlimited extent for minimizing the opening resistance because otherwise the groove would be visible due to the residual material sinking into the groove. To the extent that the foamed material layer should be at least partially retained as a supporting layer for the decorative layer, no weakening of the decorative layer can take place. However, for more robust foils, it is necessary that these be weakened because otherwise the tearing force required would be too great. In order to weaken the foil, it is mandatory that the supporting foamed material layer in the area of the groove be completely removed, and the decorative layer will sink into the groove over the course of time even when the decorative layer is weakened only slightly.
The basic idea of retaining the foamed material layer as a supporting layer for the decorative foil in the area of the line of weakening leads to the idea of forming the line of weakening by means of blind holes or pocket holes.
DE 196 36 429 C1 describes the advantages of a line of weakening produced by removal of material to form pocket holes:
1. Whereas only the residual wall thickness in the cut joint (continuous groove) can be varied as a geometric quantity for influencing the opening resistance when the line of weakening is produced by incision (described, e.g., in EP 0 711 627 A2), the width of the bridging material or web remaining between the pocket holes can be varied effectively in the case of a succession of pocket holes (perforation line).
2. When material is removed in the form of a cut line, the residual material must be thick enough or strong enough that it does not sink into the cut joint and thus become visible. With the perforation line, this sinking in is prevented, even when the residual wall thickness is smaller, by the webs which remain between the pocket holes and which act as supports.
3. A tearing resistance which is constant along the entire perforation line can also be achieved when the pocket holes have different residual wall thicknesses that are repeated periodically.
The generation of pocket holes of different residual wall thickness is based on the idea that small residual wall thicknesses and narrow webs can lead to intense thermal loading, as a result of which, as with material fatigue due to aging, the residual material sinks into the pocket holes so that the line of weakening becomes visible. In order to prevent this, material removal is carried out while periodically changing to different depths in the decorative layer. At shallower depths, the widths of the webs remaining in the foamed material layer are appreciably greater and there is less thermal loading of the decorative layer. The risk of visibility is improved with only a slight increase in the tearing resistance.
However, as tests have shown in practice, the selected spacing of the pocket holes penetrating into the decorative layer must be very large so that a web of foamed material is actually maintained. The radiation energy needed for the removal of the substrate layer and decorative layer is so high that the foamed material layer burns copiously around the pocket hole. However, as a result of the large spacing between the pocket holes, particularly when using more robust materials for the decorative layer, the required opening force is excessive.
Both of the solutions presented above are compromises between the smallest possible residual wall thickness and a small web width so as to keep the opening resistance of the line of weakening low on the one hand and the residual wall thickness and web width sufficiently large on the other hand so that the line of weakening is not visible to the passengers.
DE 195 40 563 A1 shows an instrument panel for motor vehicles which is overlain by a foil and has an integrated airbag door. The foil (decorative layer) has a line of weakening formed by a row of perforation holes which completely penetrate the material. The inventive idea in this solution consists in reducing the visibility of the line of weakening by covering the line of weakening with a laser-treated track.
Again, with the aim of generating an invisible line of weakening, it is suggested in DE 196 36 428 A1 to generate a line of weakening which alternates with the actual predetermined breaking line and is similar to the surface structure of the detector layer. (In the solutions mentioned above, the line of weakening leads over the same line as the predetermined breaking line.)
The last two solutions mentioned above require higher expenditure with respect to apparatus and technique as well as longer cutting times. Invisibility is forfeited for the sake of a low opening resistance which can best be achieved by means of a through perforation. However, this is again produced by superimposing a second laser line which makes the line of weakening “disappear” or by a line which is hardly visible in the surface.